The present invention relates to a cermet alloy of titanium-carbide (hereinafter referred to as TiC) base and, more particularly, to a TiC-base cermet alloy which is increased in both strength and toughness by strengthening its binder phase.
Currently, an alloy of titanium-nitride (hereinafter referred to as Ti(C, N)) base containing nitrogen is mainly used as a cermet alloy for machining tools. Ti(C, N)-base cermet alloy is improved in room-temperature strength, oxidation resistance and machinability in comparison with a conventional TiC-base cermet alloy.
In both of the TiC-base cermet alloy and the Ti(C, N)-base cermet alloy, the hard phase is constituted of particles of a core structure which comprises a core portion (TiC, Ti(C, N), respectively) and a peripheral portion ((Ti, Mo)C, (Ti, Mo)(C, N), respectively) surrounding the core portion. Particles of the Ti(C, N)-base cermet alloy are refined due to containing nitrogen, and thus, its room-temperature strength is improved.
Also, it is publicly known that the Ti(C, N)-base cermet alloy is excellent in high-temperature strength
High-temperature strength of a TiC-Mo.sub.2 C-Ni alloy containing nitrogen is explained in detail in "Fine Particles and Powder Metallurgy", Vol. 30, No. 3, April, 1983, saying that this alloy has an excellent high-temperature strength because a larger amount of Mo is dissolved in the binder phase so as to suppress dynamic recovery of the binder phase. Although the process of soluting of Mo into the binder phase is not mentioned in the above document, it can be presumed as follows: titanium, which is over saturated as a result of denitrification of Ti(C, N) at the time of vacuum sintering, is combined with carbon of Mo.sub.2 C to form a carbide, and the remainder molybdenum is dissolved into the binder phase.
This document also discloses that the large the nitrogen content is, the more molybdenum will be dissolved in the binder phase, and that the larger the amount of addition of Mo.sub.2 C, which is supplying source of Mo, is, the more molybdenum will be dissolved. For instance, in an alloy whose composition by wt. % is TiC.sub.0.7 N.sub.0.3 -11%Mo.sub.2 C-24%Ni, the amount of Mo in the binder phase is 3.4 wt. % (the amount of dissolved Ti is 10.6 wt. %), in an alloy whose composition by wt. % is TiC.sub.0.7 N.sub.0.3 -19%Mo.sub.2 C-24%Ni, the amount of Mo is 10.0 wt. % (the amount of dissolved Ti is 9.3 wt. %), and in an alloy whose composition by wt. % is TiC.sub.0.7 N.sub.0.3 -27%Mo.sub.2 C-24%Ni, the amount of Mo is 8.2 wt. % (the amount of dissolved Ti is 5.7 wt %). On the other hand, in an alloy whose composition by wt. % is TiC-11%Mo.sub.2 C-24% Ni-containing no nitrogen, the amount of Mo is 0.2 wt. % (the amount of dissolved titanium is 12.7 wt. %), and in an alloy whose composition by wt. % is TiC-19%Mo.sub.2 C-24%Ni containing no nitrogen, the amount of Mo is 1.8 wt. % (the amount of dissolved titanium is 16.3 wt. %). It can be understood from this result that, in the case of the TiC-base cermet alloy containing no nitrogen, it will be extremely difficult to strengthen the binder phase even if the amount of addition of Mo.sub.2 C is increased.
As described above, the Ti(C, N)-base cermet alloy has better properties in comparison with the TiC-base cermet alloy. However, it involves some problems, for example, changes are caused between the properties of the surface and the inner portion of the cermet alloy and pores are likely to be formed in the structure when the cermet alloy is denitrified at the time of vacuum sintering. If pores are formed in the structure, it is impossible to obtain the strength which the Ti(C, N)-base cermet alloy is originally supposed to have.
With relation to this problem, there have been made various proposals such as introduction of nitrogen gas in the sintering environment, and control of temperature elevating conditions (e.g., JP-A-2-93036, 2-145741 and so forth). However, these suggestions are not favorable in respect of the productivity.
Moreover, as compared with the TiC-base cermet alloy, the Ti(C, N)-base cermet alloy has higher hardness but is inferior in toughness. In relation to this problem, there have also been made various proposals for solving it. The inventors of the present application proposed a method of adding WC independently and separately from a solid solution with Ti(C, N) in JP-A-63-83241. However, essential improvement has not been accomplished in by JP-A-63-83241.